Multiecho water‐fat separation and simultaneous R estimation with multifrequency fat spectrum modeling

Abstract

Multiecho chemical shift-based water-fat separation methods are seeing increasing clinical use due to their ability to estimate and correct for field inhomogeneities. Previous chemical shiftbased water-fat separation methods used a relatively simple signal model that assumes both water and fat have a single resonant frequency. However, it is well known that fat has several spectral peaks. This inaccuracy in the signal model results in two undesired effects. First, water and fat are incompletely separated. Second, methods designed to estimate T* 2 in the presence of fat incorrectly estimate the T* 2 decay in tissues containing fat. In this work, a more accurate multifrequency model of fat is included in the iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) water-fat separation and simultaneous T* 2 estimation techniques. The fat spectrum can be assumed to be constant in all subjects and measured a priori using MR spectroscopy. Alternatively, the fat spectrum can be estimated directly from the data using novel spectrum self-calibration algorithms. The improvement in water-fat separation and T* 2 estimation is demonstrated in a variety of in vivo applications, including knee, ankle, spine, breast, and abdominal scans. Key words: water-fat separation; R* 2 measurement; T * 2 measurement; fat spectrum; fat quantification; fat spectral peak Multiecho chemical shift-based water-fat separation methods have seen a recent increase in clinical use (1-6), particularly in challenging applications where inhomogeneous magnetic fields cause failure of conventional fat saturation methods. Dixon (1) first used in-phase (IP) and out-of-phase (OP) images to analytically calculate the water and fat images, in the so-called the 2-point Dixon method. Glover (2) and Glover and Schneider (3) then extended the idea to collect three echoes such that the water-fat separation can be performed with the correction for B 0 field inhomogeneity. In the last decade, numerous variations have been proposed based on the 2-point and 3-point Dixon methods. These previous methods assumed a relatively simple signal representation that models both water and fat as a single resonant frequency. For most applications, this is a satisfactory model and excellent qualitative water-fat separation can be achieved.Although water is well modeled by a single frequency, this is not true for fat. In general, it is assumed that fat resonates at a single frequency ϳ3.5 ppm downfield from water (approximately 210 Hz at 1.5T, and 420 Hz at 3T). However, it is well known that fat has a number of spectral peaks (7-17). In particular, the spectral peak from olefinic proton (5.3 ppm) is close to the water resonant frequency, which will manifest as a baseline level of signal within adipose tissue on the separated water images (2,14,16). This effect is also commonly seen on images acquired with either conventional fat saturation (18) or spatial-spectral excitation (19). In general, this small signal within the fatty tissues is cl...

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